About query

Hi, in the post http://www.elasticsearch.org/tutorials/2011/07/18/attachment-type-in-action.html
,
Attachment Type in Action ,

curl -X PUT "localhost:9200/test/attachment/_mapping" -d '{
"attachment" : {
"properties" : {
"file" : {
"type" : "attachment",
"fields" : {
"title" : { "store" : "yes" },
"file" : { "term_vector":"with_positions_offsets",
"store":"yes" }
}
}
}
}
}'

curl "localhost:9200/_search?pretty=true" -d '{
"fields" : ["title"],
"query" : {
"query_string" : {
"query" : "amplifier"
}
},
"highlight" : {
"fields" : {
"file" : {}
}
}
}'

in here, what is function for "fields" : ["title"] , the query
also can search the content of an attchment .